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This review first appeared in the July 2010 issue of hi-end hifi magazine High Fidelity of Poland. You can also read thisreview of the Tenor Audio 175S in its original Polish version. We publish its English translation in a mutual syndication arrangement with publisher Wojciech Pacula. As is customary for our own reviews, the writer's signature at review's end shows an e-mail address should you have questions or wish to send feedback. All images contained in this review are the property of High Fidelity or Tenor Audio. Ed.

Tenor Audio the brand is barely known in Poland if at all. While I did write about it a few times, that was only in the informal context of covering Munich's High End show. There Tenor amplifiers played in systems set up by Kharma’s German distributor. It was the first time I heard that those loudspeakers could actually perform at all. All previous presentations had been absolutely unsatisfactory - bright, mechanical and lifeless. With the Canadian amps, everything was suddenly fine. I listened I think to the 175S stereo amplifiers, one per side, big powerful units weighing 54kg (67kg with packaging) and built as hybrid circuits with tube voltage-gain stages and transistor current buffers.

Alas, Tenor did not start with transistors. The 175S and 350M monos came to life by virtue of evolution. This followed a path I too have trod to be familiar with. Their goal was creating a high-power amplifier able to work across very wide bandwidth (not to be confused with ‘wideband’ which describes loudspeakers using one driver to cover the audible spectrum) and into big loudspeakers. The company mottos Power. Performance. Passion and Ultimate Audio Amplification nicely summarize that. According to Tenor, they couldn’t speak of ‘ultimate’ amplification until recently. The company began its power adventures with OTL output-transformer-less tube amplifiers. At the beginning of the 21st century they developed a proprietary OTL circuit called Tenor OTL.

This became so successful that according to company materials 10 years later those original amplifiers fetch a 50% higher price on the second-hand market than they sold for new. From the very beginning, Tenor was focused on delivering high power. The OTL amplifiers offered 75 watts per channel, a lot for tubes but insufficient for Tenor’s ultimate ambitions. So they built prototypes of even more powerful versions whose sheer size and heat emissions caused issues with user and speaker protection. That project was terminated and the next logical step became the hybrid approach.

A hybrid amplifier combines valve and transistor stages, usually with the tubes on the input and transistors or ICs at the output (the latter as for example in the Synthesis Magnum 50/100 Series or FatMan’s iTube Carbon 2). Rarely do machines combine solid-state input sections with tube output stages but some exist – KR Audio’s reviewed Kronzilla SXI-S for example and the IA-45 by Polish company Linear Audio Research [Nagra’s new 300B amplifier will also fall into this group – Ed]. Because 99% of hybrid circuits follow the tube/transistor pattern however, let’s focus on that. The main argument also adopted by Tenor is that tubes are ideal voltage amplifiers while currents are modest. This exploits the good linearity of tubes, their resistance to clipping and ability to work with high voltages (this also is what SPL does in their studio gear company where transistors work at 120V). Transistors are best used for current gain when high reserves and drive are essential. Combining these specific strengths creates the hybrid tube/transistor amplifier.

I know devices of that type, having listened to many of them, even built some myself. It’s a very good platform for creating a nicely warm-sounding amplifier. But despite what you’ll sometimes hear claimed, this is not an ideal solution. Almost all the hybrid amplifiers I know sound too warm and muddy and suffer limited resolution. They take the worst from either technology, not the best. I appreciate the appeal—the sound is really lovely for the low and middle price ranges—but in the high end that's no longer acceptable. Hence Tenor Audio developed its own approach. According to CEO Jim Fairhead, the Canadian engineers began with the OTL concept and eventually developed it into their current input stage. It’s actually a A class OTL amplifier working with high voltages whose outputs couple to a beefy transistor circuit in unity-gain mode. That stage only provided current gain to leave voltage amplification exclusively to tubes. It’s an interesting twist.

Besides my reference system, the Tenor amp also saw the Avalon Ascendant loudspeaker; the Avid Acutus Reference turntable with SME Model IV arm, Air Tight PC-1 Supreme pickup and Avid Pulsare phono stage; and Acrolink Mexcel 7N-PC9100 and 7N-PC9300 power cords. I’d already heard an amplifier with a similar signature. It did not take long to remember which, the Italian Audia Flight 100. It’s probably no coincidence that Audia based their amplifier output stage on the unity-gain notion as well. The Flight 100 too was a big trans-impedance job using output transistors as a kind of follower (in their case 16 Toshibas capable of delivering up to 160A). The approach seems similar. Sonically Tenor Audio pursues the same direction but makes everything many times better - so good in fact that I can say theirs is the best transistor output amplifier I’ve yet heard in my system; and one of the best regardless of technology, period. On my personal Mount Everest—not fully but more about that later—live two 300B amplifier hermits, the Polish Ancient Audio Silver Grand Mono and the Combak Reimyo PAT-777 from Japan.